This invention relates to apparatus for the cryofixation of specimens, particularly natural specimens.
The cryofixation of natural specimens is commonly carried out by immersion cryofixation, or by metal mirror surface cryofixation (which is also known as slammer system cryofixation). In immersion cryofixation, the specimen is cooled rapidly from room temperature or from body temperature to a temperature below -100.degree. C. by introducing it into a cooling bath. In metal mirror surface cryofixation, the specimen is struck against a highly polished metal surface to effect the necessary cooling.
In carrying out these processes it would be desirable, particularly in the case of sensitive biological or medical specimens or technical specimens with a similarly high water or fluid content, which have not been pretreated by a chemical fixation and/or anti-freeze agent (such as sugar or glycerine), to achieve a sharp transition from room temperature or body temperature to the fixation temperature. However, this is impossible in practice, because the surface level of the refrigerant liquid or of the metal mirror surface must not be allowed to come into contact with the room atmosphere, but must always remain covered by a cold inert gas such as gaseous nitrogen. Otherwise, water vapour, CO.sub.2 and O.sub.2 would immediately be deposited upon the low temperature liquid (normally at -160.degree. C. or below) or on the deeply cooled metal mirror surface.
Frost precipitations of this type would immediately render the metal mirror surface unserviceable. And in immersion cryofixation such deposits would abruptly alter the composition of the liquid cryogen, rendering it unsuitable for cryofixation. In the case of liquid propane, which is commonly used for immersion cryofixation, there is also the possibility of an O.sub.2 accumulation which leads to an acute danger of explosion.
In order to eliminate these risks, it is not sufficient merely toi maintain a thin covering layer of dry cold gaseous nitrogen over the liquid surface or the metal mirror surface. It is in addition also necessary to dimension the depth of this protective layer so that a covering layer of gaseous nitrogen is always maintained, even when air currents are present in the room. Adequate long-term security can only be ensured in such a case if the liquid surface level or the metal mirror surface is located at approximately 15 to 20 mm below the boundary layer which the cold dry gaseous nitrogen forms with the external room atmosphere. During the injection of the specimen into the refrigerant medium or on to the metal mirror surface, it must then first of all penetrate this protective layer of 15 to 20 mm cold gaseous nitrogen before it enters the refrigerant medium itself or strikes the metal mirror surface. During this operation it is impossible to prevent unnatural modification of the surface layer of the specimen that is of interest, because due to its low thermal capacity and poor thermal conductivity the gaseous nitrogen permits deleterious cooling but does not enable rapid stabilisation to be achieved by the freezing process.